Glossy with 0 roughness gives dark reflections
You have set too low values for clamping and amount of light bounces.


In Properties editor > Render tab > Sampling rollout set Clamp Indirect to 0 (you can set it to something around 2 or 3, but test those values while changing). This will turn off clamping the values and return back to default. With that much clamping there won't be almost any reflection and light of higher values than clamping maximum.   

See more related: 


Why does this mirror, made using a glossy material, not fully reflect all the incomming light?  
Cycles: Emission Shader does not bring enough light
Why are lamps and mesh lights not illuminating anything?

In Properties editor > Render tab > Light Paths rollout set Diffuse Bounces back to 4 (default) or even higher. It will allow light bouncing from diffuse surface to be bounced more times and make diffuse surface a bit brighter.

With current settings the reflection and diffuse surface look quite dark:



With default settings for Clamp Indirect and Diffuse light paths: